The transistors on a microprocessor are connected to form very complex circuitry using copper wiring. The copper wires are insulated using a low-k dielectric material.
The low-k dielectric material is typically a hydrogenated carbon-doped silicon oxide (SiCOH). The free surfaces of the low-k dielectric material are terminated with methyl groups (CH3) bound to silicon. Processing steps, such as etching which is typically performed by reactive ion etching, effectively remove the methyl terminations. The etching process leaves either dangling bonds or hydroxyl groups (Si—OH) on the free surfaces. As a result, the film becomes more hydrophilic and can readily absorb moisture (Mikhail R. Baklanov et al., Journal of Applied Physics, 041101, p 113, 2013). This damage, so-called plasma-induced damage, in turn leads to an increase in the dielectric constant. The degree of the increase in the dielectric constant depends on the severity of the damaging process. The plasma-induced damage degrades the performance of the resulting transistor device.
Another effect of carbon depletion is its impact on critical dimensions. For instance, the etching process used to form a trench through the low-k film would tend to leave the trench walls depleted of carbon. In subsequent wet stripping or cleaning processes the trench can be significantly broadened, a problem that becomes even more critical as feature size is reduced.
Ji et al. (U.S. Pat. No. 5,814,563) disclose using a mixture of a fluorohydrocarbon, carbon-oxygen, and NH3-generating gases to achieve high selectivity of dielectric (such as SiO and SiN) to p-Si layers. Shane (US Pat App Pub No 2003/0162395) discloses addition of a nitrogen-comprising gas to a fluorocarbon to deposit a polymer on the mask to improve selectivity while etching silicon dioxide layer. Nemani et al. (US Pat App Pub No 2014/0199851) disclose using a plasma process performed by flowing NF3 and NH3 to remove the modified portion of silicon nitride layer to pattern a silicon nitride dielectric film. Hamrah et al. (U.S. Pat. No. 5,242,538) discloses using CF4 and NH3 etching gases and selectivity of silicon oxide to polysilicon of up to 100:1 was observed. Pu et al. (U.S. Pat. No. 5,843,847) also discloses adding an additional nitrogen gas to a fluorinated etching gas to assist in feature dimensional control.
Nitrogen containing compounds have been used as etching gases. For example, Khandelwal, et al. (“Dry removal technology for advanced CMOS devices”, Nanochip Tech. J., vol. 11, issue 2, 2013, p 17-19) disclose an in-situ dry removal process using NH4F as etchant. Garg et al. (US Pat App Pub No 2006/0062914) disclose an activated reactive gas to treat the surface of a substrate. Garg et al. describe at paragraph [0019] that the activated reactive gas may include a large variety of fluorine-containing gases, including C3F3N3, fluoroamines such as CF5N, fluoronitriles such as C2F3N, C3F6N, and CF3NO. Felker et al. (U.S. Pat. No. 6,508,948) disclose perfluorinated heteroaromatic amine etching compounds, including cyanuric fluoride compounds. One disclosed cyanuric fluoride compound is pentafluoropyridine C5F5N.
U.S. Pat. Nos. 6,569,774 and 7,153,779 to Trapp disclose a plasma etch process for forming a high aspect ratio contact opening through a silicon oxide layer. At least one etch gas is used that includes one or more nitrogen-comprising gases to deposit a polymeric surface material during the etching for maintaining a masking layer over the silicon oxide layer.
US Pat App Pub No 2015/0371869 to Surla et al. discloses a method for etching silicon-containing films using organofluorine compounds containing at least one C≡N or C═N functional groups.
U.S. Pat. No. 6,413,877 to Annapragada discloses a method for making an etched organo-silicate-glass (OSG) layer over a substrate. The patterned resist mask is stripped without stripping the sidewalls using a medium density plasma of N2/O2, N2/H2, or N2/NH3.
U.S. Pat. No. 6,777,344 and US Pat App Pub No 2004/0211517 to Annapragada et al. disclose a process for stripping photoresist from a semiconductor wafer formed with at least one layer of OSG dielectric and a method of etching a stack using a fluorine-containing gas and an ammonia-containing gas, respectively.
Nitrogen plasma or co-reactant (N2, NH3) with fluorocarbon gases for low-k etching/stripping are also report in several studies. See, e.g., Y. Miyawaki et al., JJAP 52 (2013) 020204; S K Yang et al., JKPS 52 (2008) 1786; H. Nagai et al., JJAP 42 (2003) L212; and X. Su, JVST B 25 (2007) 156.
Thus, a need remains for improved low k plasma etching processes, which reduce plasma-induced damage and carbon loss in the low k film during the low k etch process, while maintaining profile control and selectivity to the mask and etch stop layers.